An HMD is already known which is mounted on the head of an observer in order to project, onto the observer's eye, image light representing an image to be displayed. According to the HMD, the observer is allowed to directly observe the image without the intermediation of a screen onto which the image is projected.
In general, an HMD includes a display unit and an attachment. The display unit projects image light corresponding to an image signal onto the observer's eye and thereby displays the image to the observer. The attachment is configured to attach the display unit to a mounted member to be mounted on the head of the observer. For example, employable attachments include a frame of eye glasses, a dedicated frame of eye glasses type, a helmet, a band, goggles, and a frame to be mounted on the head of an observer by using one ear. An example of detailed form of the display unit is a single-eye display unit projecting image light onto the observing eye which is one of the observer's both eyes and thereby displaying the image to the observer.
In the prior art, there is an HMD of see-through type provided with a single-eye-type display unit. The display unit includes a half mirror arranged in front of the observer's eye. Further, Patent Document 1 discloses a technique that a protection member extending from the tip part of the half mirror toward the left and right directions relative to the observer is attached to the tip part of the half mirror and that the observer-side surface of the protection member is flattened. According to this technique, even when the tip part of the half mirror gets close to the observing eye, the protection member having a more flat shape approaches the observing eye before the tip part does. This improves the safety of the HMD for the observer's eye during the use.